A potential disruptive solution to further increase capacity in long-range optical transmission systems is to exploit more than one spatial propagation mode in optical fibers. Such mode division multiplexing enables for multiple-input multiple-output (MIMO) transmission. WO-A-2010/151432 discloses examples of such an optical multimode transmission system. In optical multimode transmission systems, the different spatial modes have different propagation constants and arrive with a time delay at the receiver side. This delay is a function of the transmission distance and the differential mode group delay (DMGD). In long-range applications, such delay is likely to exceed a processing depth capability of conventional digital signal processors.